Impact sprinklers have been used since the 1930's for distributing water, for instance, in agricultural irrigation. A typical impact sprinkler utilizes a discharge member or deflector directing water into a spoon connected to an impact arm. The impact arm is connected to a torsion spring biasing the spoon towards the water stream such that the spring absorbs a portion of the kinetic energy and momentum of a portion of the water stream as the water strikes the spoon. The water strikes the spoon for a period of time while also causing the spoon to be moved away from the water stream by rotating around a generally vertical axis. In doing so, the shape of the spring is changed from its natural position, thereby storing potential energy and providing a return bias force.
The momentum of the moving spoon causes the spoon and impact arm to move completely away from the water stream, at which time the water is free to expel unimpeded. However, in the absence of water contacting the spoon, the stored energy of the spring is expelled by directing the spoon back toward the water stream. The amount of time during which the spoon is not being contacted by the water stream is known as the dwell time.
As the spoon and impact arm return, the spoon once again passes through the water stream. Because the impact arm and the structure to which it is connected have mass and, therefore, inertia, the return of the impact arm strikes the structure to which the deflector is connected. This striking causes the discharge member and its associated structure to rotate a short distance around the generally vertical axis in the direction of the return of the impact arm. However, the water stream once again strikes the spoon such that the spoon and impact arm are moved out of the stream and against the bias of the spring, and the process is repeated.
During the dwell time, the water stream is free to expel unimpeded. However, in such a state, the water stream takes a short time period to build up maximum throw distance. That is, the presence of the spoon in the stream causes a shortening of the distance to which the water stream may expel. When the spoon is moved out of the water stream, there is a time period required for the water to reach the distance which can be achieved with continued absence of interference. Though this time period is relatively short, it is common for the spoon to return to an interference position before the water stream is able to achieve a maximum distance. This reduces the coverage area of the sprinkler and concentrates the water in a smaller area.
The coverage area of the sprinkler is also influenced by the discharge member, such as a nozzle discharge. Typically, the nozzle discharge expels the water at a fixed trajectory angle. In the absence of the spoon and once the water stream reaches its maximum distance very little water will be spread at shorter distances. In such a system, it is only by virtue of errant spray and the spoon interfering with and slowing down the water stream that water is deposited short of the maximum distance. The ability to change water trajectory is afforded by changing out the entire sprinkler for another sprinkler with a different nozzle discharge trajectory.
The energy directing the spoon out of the water stream, or drive energy, is stored in the torsion spring. However, friction between moving parts wastes a portion of the drive energy. It is common for the impact arm and its structure to be supported by a lower thrust bearing member or surface that contacts a sprinkler body or the rotating shaft and nozzle portion. This friction reduces the efficiency in transferring energy from the kinetic energy of the water stream to potential energy in the torsion spring.
To maximize dwell time, the impact arm should pass as far out of the water stream as possible. To achieve this, the impact arm is given a high mass while the torsion spring is given a low spring constant, and the spring is then referred to as a light spring.
One way of increasing dwell time would be to remove the lower thrust bearing. In the absence of the lower thrust bearing surface, the impact arm and its structure must be supported, most commonly by hanging the structure from its torsion spring. However, the torsion spring in such a system requires a sufficient size to support the mass of the impact arm and its structure. This sacrifices the amount that the spring is able to deform due to the deflection before all the energy is converted to potential energy. Accordingly, the impact arm ceases moving away from the water stream and begins to return towards the water stream. Consequently, dwell time is reduced as the impact arm returns quickly, and the overall impact frequency is high. Therefore, the water stream is not able to achieve the maximum distance.
Another shortcoming encountered with impact sprinklers is the variation in performance of the sprinkler under varying water pressures. More specifically, a sprinkler has a range of pressure under which quality performance is achieved. Outside of that range, the sprinkler suffers from poor performance, such as by rotating erratically or spinning rapidly out of control.
Water pressure can be affected by a number of factors, such as the source pressure, the pressure created by the water through the nozzle, and the shape of the discharge member. In order to avoid the sprinkler rotating erratically or spinning rapidly out of control and to optimize the performance characteristics of the sprinkler, the rotation time should be relatively constant or within a narrow range under different water flow and pressure characteristics.
One approach to control the rotation time of the sprinkler under varying water pressure utilizes a water-pressure actuated braking mechanism. Generally, this braking is done by using a stack of washers and a compression spring located against the previously mentioned lower thrust bearing member or surface. The washers are located in the water stream and below the point at which water enters the sprinkler. More specifically, the term sprinkler refers generally to the sprinkler head that includes threads on its lower end for securing to a stem or pipe that delivers water from the water source. The rotating portion of the impact sprinkler includes the nozzle entry, which is in turn located adjacent the washers. The washers are located within or below the threads of the sprinkler and the water pressure forces the washers against the sprinkler nozzle to form a dynamic seal with the moving nozzle.
In this type of braking system, braking force increases with increased water pressures. In addition, as the braking force increases, so does the drive energy. That is, the energy stored by the torsion spring for returning the impact arm returns. Though the impact frequency does not significantly increase, the angular distance traveled by the rotating part of the sprinkler including the nozzle discharge for each impact increases such that the time for a single rotation to be completed by the sprinkler, known as the rotation time, decreases. As the rotation time decreases, the distance achieved by the water stream decreases, and the water stream begins to tail. Because of these, the range of operating pressures that provide quality performance narrows.
The described braking system utilizing washers and a dynamic seal only recognizes pressure and not flow rate. This is because the washer stack is positioned in the flow of water from the stem, prior to the water passing through the nozzle, an arrangement typically necessitated by using the nozzle and water discharge as a single component which must be permitted to rotate with the rotation of the direction of the water stream. However, when nozzles or nozzle discharges with different flow rates are used, the pressure may vary differently, or not at all. Accordingly, this braking system cannot control the rotation time under different flow rates, which results in a varied rotation time. The varied rotation time limits a sprinkler to provide optimal performance only over a smaller or narrower range of water flow specification.
An important characteristic of the systems as described is the use of bearings and braking surfaces that rely on friction. As is known, friction has a cumulative negative effect on the life and performance of a sprinkler. It is also known that water commonly used in agricultural settings contains debris including, for instance, sand, rocks, dirt, and volcanic particles. In the described thrust bearing and washer brake configurations, this debris can become lodged between the surfaces and accelerate the wear on the moving parts. Furthermore, grit can enter the dynamic seal formed by the washers and bind the mechanism.
Despite the large-scale applications for which impact sprinklers are used, these systems still utilize relatively fragile components susceptible to damage and external interference. For instance, it is known that weeds or proximally growing verdure and brush can grow into the sprinkler mechanism, thereby clogging the mechanism and preventing its proper operation. In addition, it is known that accidental external striking of the sprinklers, such as by dropping a sprinkler, can occur and cause damage.
Accordingly, there is a need for improved rotary impact sprinklers.